This invention relates to aluminous abrasive grits and particularly to seeded sol-gel alumina abrasive materials with improved grinding performance. Such aluminas are made up of sub-micron alpha alumina crystals and the abrasive grits are sintered to essentially full theoretical density, that is in excess of 95% of the theoretical figure for alpha alumina.
Seeded sol-gel alumina abrasives are conventionally produced by uniformly dispersing sub-micron sized particles of alpha alumina particles in an aqueous dispersion, such as a sol or gel, of an alpha alumina precursor, (which is usually but not essentially, boehmite). The water is then removed and the mixture is fired to above the conversion temperature at which alpha alumina is formed and sintered to essentially theoretical density. A simple seeded sol-gel process is described for example in U.S. Pat. No. 4,623,364.
The firing operation performs the tasks of converting the transitional alumina forms present in the dried gel to the alpha form and the sintering of the alpha alumina closes up residual porosity and ensures that the particles have adequate density and hardness to function well as abrasive grits. It is known that excessive time at sintering temperatures, which are generally between 1300.degree. and 1400.degree. C. for seeded sol-gel materials and about 100.degree. C. higher than that for unseeded sol-gel aluminas, can lead to crystal growth. Since crystal growth is generally regarded as undesirable because it is associated with loss of abrasive properties, it is often considered appropriate to incorporate into the gel certain oxides which act to limit the crystal size growth. U.S. Pat. No. 4,314,827 teaches magnesia and zirconia for this purpose in connection with unseeded sol-gel processes, and U.S. Pat. No. 4,623,364 adds silica, zirconia and chromia to the list for seeded sol-gel processes. In spite of the above teaching U.S. Pat. No. 4,314,827 for reasons that are unspecified in the patent, positively excludes significant amounts of calcia and alkali metal oxides from the scope of the invention claimed. In more recent times it has been disclosed that products made with the incorporation of minor amounts of rare earth metal oxides, yttria, transition metal oxides and lithium oxide bring advantageous results whether the gel is seeded or unseeded. See for example U.S. Pat. Nos. 4,770,671; 4,881951; 5,188,908; 5,190,567; 5,192,339; 5,215,551; 5,387,268; and 5,403,795 and European Applications 408,771; 594,455; 561,865; and 622,438.
It has now been found that the amount of such minor components should be controlled within narrow limits to obtain the best abrasive alumina grits by a seeded sol-gel process. This is somewhat surprising in view of the many patents teaching the positive advantages of the incorporation of minor amounts of oxides to modify the grain boundaries of the crystal structures or form a separate phase at such boundaries.